Synthetic silica glass has characteristics such that it is a material transparent over a wide range of wavelength region from a near infrared region to a vacuum ultraviolet region, the thermal expansion coefficient is very small, it is excellent in dimensional stability, and it contains no substantial metal impurities and is highly pure, and it has hence been used mainly for optical components for optical apparatus wherein conventional g-line or i-line is used as a light source.
In recent years, along with high integration of LSI, a technique for fine patterning with a narrower line width has been required in the lithographic technology for drawing an integrated circuit pattern on a wafer, and for this purpose, shortening of the wavelength of the light source for exposure is being advanced. Namely, for example, the light source for a lithographic stepper has been advanced from conventional g-line (wavelength: 436 nm) or i-line (wavelength: 365 nm) to adopt a KrF excimer laser (wavelength: 248 nm) or an ArF excimer laser (wavelength: 193 nm) and the optical component to be used for the above-mentioned stepper is required to have light transmittance, stability and durability up to a short wave region of about 190 nm.
When conventional synthetic silica glass is irradiated with a high energy beam of e.g. a KrF or ArF excimer laser, new absorption bands will be formed in an ultraviolet region, and thus, it has a problem as an optical component for constituting an optical system wherein such an excimer laser is used as a power source. Namely, when it is irradiated with the above laser for a long period of time, an absorption band at about 215 nm due to E′ center and an absorption band at about 260 nm due to NBOHC (non-crosslinking oxygen radical) will be formed.
It is considered that such absorption bands are formed, as inherent defects due to oxygen deficient type defects such as ≡Si—Si≡ and oxygen excessive type defects such as ≡Si—O—O—Si≡ in the synthetic silica glass undergo photo-reactions by irradiation with a laser light. These absorption bands will cause deterioration of the transmittance, increase of the absolute refractive index, change in the refractive index distribution or generation of fluorescence.
Various methods have been studied as methods for solving these problems, and it is known to be effective to incorporate hydrogen molecules in synthetic silica glass. For example, JP-A-3-88742 discloses a method wherein synthetic silica glass is permitted to contain hydrogen molecules in an amount of at least 5×1016 molecules/cm3 and OH groups in an amount of at least 100 ppm, to increase the durability to ultraviolet light.
However, with the synthetic silica glass containing OH groups in an amount of at least 100 ppm and hydrogen molecules in an amount of at least 5×1016 molecules/cm3 in JP-A-3-88742, there has been a problem such that when irradiated with a laser light, it generates a red color fluorescence of 650 nm, although formation of the absorption band at about 215 nm may be suppressed, and improvements may be obtained with respect to deterioration of the transmittance, increase of the absolute refractive index or change of the refractive index distribution when irradiated with an ArF or KrF excimer laser light for a long period of time. This red color fluorescent band of 650 nm is simultaneously accompanied by an absorption band at 260 nm and thus creates a serious problem especially when it is used for an optical component of an apparatus wherein a KrF excimer laser having a wavelength close thereto is used as a light source.
As a means to dope hydrogen molecules, JP-A-1-201664 discloses a technique for carrying out heat treatment at a high temperature at a level of from 800 to 1000° C. Further, JP-A-6-166522 discloses a technique to incorporate hydrogen at a concentration of at least 1×1017 molecules/cm3 into synthetic silica glass by maintaining it at a low temperature of a level of from 300 to 600° C. in a hydrogen-containing atmosphere under a pressure of at least 1 atm, since if hydrogen doping is carried out at a high temperature at a level of from 800 to 1000° C., reduction type defects due to hydrogen will form in the synthetic silica glass. Specifically, it is concluded that it is preferred to carry out hydrogen doping at 100 atms and 1 atm, respectively, and it is particularly preferred to carry out hydrogen doping under a high pressure of at least 50 atms.
Whereas, in the case of a stepper wherein a KrF excimer laser is used as a light source, it is advisable to control the decrease in the transmittance of light of 248 nm after irradiation of 106 shots of a KrF excimer laser light under conditions of an energy density of 400 mJ/cm2/Pulse and a frequency of 100 Hz, to be at most 0.1%. For this purpose, it is necessary to control the change in absorption coefficient at about 215 nm (e.g. 214 nm) when a KrF excimer laser light is irradiated under the above conditions, to be at most 2.1×10−2. In JP-A-6-166522, the decrease in transmittance is measured by irradiating 5×106 shots of a KrF excimer laser light under conditions of an energy density of 150 mJ/cm2/Pulse and a frequency of 100 Hz, but by such a measuring method, the energy density of the KrF excimer laser light is so small that it is not adequate as a measuring method in recent years where higher excimer laser resistance is required.
The present inventors have conducted further detailed studies on the hydrogen molecule doping method by using a KrF excimer laser having a higher energy density and as a result, have found that even if hydrogen is doped under a high pressure as recommended in JP-A-6-166522, it is not necessarily possible to obtain a synthetic silica glass having adequate laser resistance.
Namely, in the case of a silica glass containing hydrogen molecules in an amount of at least 1×1017 molecules/cm3, formation of hydrogen bond type defects such as ≡Si—H and oxygen deficient type defects such as ≡Si—Si≡ (hereinafter, ≡Si—H and ≡Si—Si≡ will be referred to as reduction type defects) by carrying out treatment at 500° C. under a hydrogen pressure of 100 atms, has been confirmed by the measurement by e.g. Raman spectrometry or vacuum ultraviolet spectrometry, and further, under irradiation with a laser light, a substantial absorption peak has been observed at about 215 nm. This strong absorption peak at about 215 nm has a skirt extending over a wide wavelength region of from 180 to 250 nm and hence is problematic when used for an optical component of an optical apparatus wherein an ArF excimer laser or a KrF excimer laser is used as a light source.
It is an object of the present invention to provide a process for producing a synthetic silica glass optical component, whereby a synthetic silica glass optical component having substantially no reduction type defects and containing hydrogen can be obtained with good productivity.
Further, it is an object of the present invention to provide a synthetic silica glass optical component which is free from generation of fluorescence or decrease in transmittance even when irradiated by an excimer laser light, and a process for its production.